1. Field of the Invention
The present invention relates to a rotary information recording/reproducing apparatus for turning and positioning a head to a predetermined point on an information recording disk and, more particularly, to a rotary information recording/reproducing apparatus which has a smaller size and can position a head at a high speed and with high accuracy.
2. Description of the Prior Art
A prior art magnetic disk apparatus will be described, taking U.S. Pat. No. 5,027,242 as an example. Information recording disks arranged one above another are driven to rotate at a constant speed by a disk rotating motor fixed to a base. Heads are supported by carriage guide arms through head suspensions in such a manner that each head is floated with a small gap held relative to an associated disk for recording/reproducing information. High-speed and highly accurate positioning of the head is performed by rotatively driving the carriage with a voice coil motor. The carriage is mounted and supported through ball bearings on and by a pivot shaft fixed to the base such that the carriage can rotate about the pivot shaft. The voice coil motor comprises a voice coil, permanent magnets and a yoke. 0f these components, the voice coil is attached to the carriage, whereas the permanent magnets and the yoke (both serving as parts of a magnet-yoke assembly) are fixed to the base. Since the voice coil surrounded by the magnet-yoke assembly in sandwiched relation is under the action of a magnetic field, the carriage is rotatively driven when the voice coil is supplied with electric power.
However, when a driving force is generated in the voice coil with the supply of electric power to the voice coil, a resultant reaction force acts on the magnet-yoke assembly of the voice coil motor to vibrate the base, thereby causing vibration excitation of the disk. Also, with the generation of the driving force in the voice coil, translational excitation force acts on the carriage pivot shaft at the same time, thereby vibrating a carriage supporting system. A resultant vibration force acts on the disk driving motor to vibrate it, and hence the disk is further vibrated.
In view of the above problems, the prior art disclosed in Japanese Unexamination Patent Publication No. 1-222647 and U.S. Pat. No. 4,620,252, for example, has been proposed. The prior art apparatus includes two flat type driving coils attached to a carriage on opposite sides of the carriage pivot shaft axis. Two sets of magnet-yoke assembly are arranged in non-contact relation to the driving coils for rotating the carriage by a couple. This arrangement is intended to prevent the translational excitation force from acting on the carriage pivot shaft and to reduce vibration of a structural member caused by the carriage motion.
According to the prior art disclosed in the above-cited Japanese Unexamination Patent Publication No. 1-222647 and U.S. Pat. No. 4,620,252, however, an information recording disk having a sufficiently large outer diameter cannot be accommodated in a housing of the apparatus, making it difficult to provide the apparatus with a large capacity. Specifically, because the two driving coils are attached to the carriage in opposite relation with respect to the axis of the carriage pivot shaft and the magnet-yoke assemblies are arranged around the driving coils, the axis-to-axis distance between the carriage pivot shaft and a disk rotating shaft or spindle must be set to be greater than required in other conventional apparatus using one driving coil, in order to avoid interference between the carriage and the disk. If a disk having a conventional large outer diameter is installed in the apparatus housing, the size of the apparatus must be increased in the longitudinal direction thereof.
Further, in order to satisfy form factor dimensions such as 1.3, 1.8, 2.5, 3.5 or 5.25 inches which are standardized for small-sized magnetic disk apparatus, the disk size must be reduced to such an extent as not departing from the apparatus dimension in the longitudinal direction. This leads to a reduction in storage capacity of the apparatus. In addition, the greater axis-to-axis distance as discussed above enlarges the size of a carriage guide arm, resulting in the increased inertial moment of rotating members. This requires a longer access time and, if the output of a carriage driving motor is increased as measures for avoiding such a drawback, the apparatus size would be necessarily enlarged. Thus, it has been difficult to provide a mechanism which can maintain the apparatus within the standardized form factor dimension, to suppress vibration imparted to a disk via a base or carriage to the extent possible, and to position the head at a high speed and with high accuracy.